If I have a British recipe that lists "1 tin chopped plum tomatoes," how many ounces is that? In British English, does "tin" usually refer to a specific size? In the US, cans generally come in 14 and 28oz sizes - which one is likely closest to what the recipe means?
It is likely the same as a 14 oz can of tomatoes.
From the Wikipedia article on Tin Cans:
A standard size tin can is roughly 400g; however, the weight can vary between 385g and 425g, depending on the density of the contents.
UK shops selling tins of tomatoes quote 390-400 grams based on what I'm seeing.
This is equivalent to 14 oz. American cans will generally say the weight in ounces and in grams if you want to double check.